1. Field of the Invention
The present invention relates to deployment of the Asymmetric Digital Subscriber Line Plus (ADSL+) service which is a technology for asymmetric bi-directional data transfer over twisted pair copper wires similar to the Asymmetric Digital Subscriber Line (ADSL) service but which uses an extended downstream bandwidth in order to achieve higher downstream bit rates.
2. Description of the Related Art
Such an ADSL+ service is already known from the Contribution to the ETSI Standards Subcommittee TM6 entitled “Introduction of ADSL+”. This ETSI Contribution was authored by Krista S. Jacobsen and Arthur Redfern from Texas Instruments and has the reference TM6 022t25. In this publication, ADSL+ is presented as solution to provide higher downstream bit rates on short loops and compared to Very High Speed Digital Subscriber Line (VDSL) service. By using the frequency spectrum between 1104 kHz and 2208 kHz and doubling the number of carriers compared to traditional ADSL, downstream bit rates above 10 Mbps can be achieved with ADSL+ on loops between 1.5 km and 2.7 km in length. Depending on the loop noise, even higher bit rates in the order of 20 Mbps are achievable for loops up to 1.8 km. On longer loops however, the extra carriers in the higher frequency spectrum between 1104 kHz and 2208 kHz are too attenuated as a consequence of which ADSL+ service automatically reverts to traditional ADSL service.
Compared to VDSL, ADSL+ has the benefit that it can be deployed using the same hardware platform as ADSL: identical power requirements, comparable line driver and digital module. As a result, ADSL+ offers a cost attractive possibility to operators to upgrade their not yet depreciated ADSL infrastructure in order to be able to provide higher downstream bitrates for applications such as video streaming, and audio/video/software downloads.
To maximize the customer base that can benefit from the high downstream bit rate, ADSL+ can be deployed from remote cabinets, located closer to the customer premises (within 2.5 km from the customer) than the central office (typically up to 4.5 km distance from the customer) for downstream bit rates of 1 Mbps. However, since ADSL+ deployed from a remote cabinet is not spectrally compatible for a certain service capability with ADSL deployed from the central office (certain areas which are reachable from the central office for the low bit rate ADSL service, are not reachable for a high bit rate ADSL+ service from the central office as a result of which ADSL+ has to be deployed from a cabinet in the field), high level crosstalk effects between wires in the same bundle can drastically degrade the achievable downstream bit rates for the ADSL service deployed from the central office. An obvious solution to this problem proposed in the above mentioned ETSI Contribution is Power Spectral Density (PSD) shaping: rather than allowing ADSL+ to transmit at the maximum PSD, the transmit PSD is reduced for carriers below 1104 kHz. A drawback of spectral shaping below 1104 kHz is that part of the downstream bit rate of both the ADSL+ and ADSL services is sacrificed. Furthermore, the required PSD reduction will depend on various parameters such as the difference in loop lengths used for ADSL and ADSL+, the noise on the loop, distance between the ADSL central office and ADSL+ remote cabinet, etc., and optimal PSD reduction (i.e. maximizing the downstream achievable bit rates for the ADSL and ADSL+ services) will be frequency dependent. Therefore, PSD shaping will increase the operational and implementational complexity of ADSL and ADSL+ equipment significantly. Making the PSD shaping dependent in the distance between the central office and the cabinet represents a significant operational burden to an operator, rendering the solution known from the ETSI contribution unattractive.